This invention relates generally to cooling systems for nuclear reactors, and, more particularly, to an apparatus and method for suppressing the formation of vortices in coolant fluid circulating within such reactors.
Generally, nuclear reactors include a cylindrically reactor vessel having a hemispherical lower end. Within the reactor vessel is the reactor core, supported by a main core support. The main core support is connected to the interior walls of the reactor vessel at or adjacent to the area where the cylindrical and hemispherical portions of the reactor vessel meet. Below the main core support, the hemispherical vessel defines a lower head, or lower plenum. A generally cylindrical downcomer surrounds the reactor core. Coolant fluid, typically water, is pumped into the downcomer. The coolant fluid circulates downwardly into the lower plenum. The hemispherical shape of the lower plenum assists in evenly circulating the coolant fluid therein. A plurality of reactor core coolant inlet openings are located on the underside of the main core support. Coolant flows from the lower plenum, into the core coolant inlet openings and upwardly into the core to cool the fuel assemblies.
In order to maintain adequate and uniform cooling throughout the core, it is important that a uniform coolant flow and pressure be maintained across all of the reactor core coolant inlet openings. Non-uniform coolant pressure or flow causes uneven coolant flow into the core, which results in uneven cooling of the fuel assemblies of the core. Uneven fuel assembly cooling may force the entire core to be derated to accommodate "hot assembly" locations. Non-uniform coolant flow and pressure may result if vortices or other flow disruptions form in the coolant fluid circulating in the lower plenum.
It is desirable to provide core monitoring instrumentation within the core of a nuclear reactor. Traditionally, the leads connecting such instrumentation to the exterior of the reactor exit the reactor vessel through a central portion of the hemispherical portion of the reactor vessel. A plurality of conduits carry the instrumentation lines through the lower plenum.
The presence of the conduits in the lower plenum assists in maintaining even coolant flow within the lower plenum and disrupting the formation of vortices in the circulating coolant fluid. Such vortices disrupt coolant flow and produce low pressure areas at the core coolant inlets which they intersect.
In newer reactors, it has become desirable for any instrumentation conduits to exit the reactor vessel other than through the lower plenum. It has been found that the absence of instrumentation conduits from the lower plenum permits vortices to form in the circulating coolant in the lower plenum. Accordingly, there is a need for a simple and inexpensive apparatus and method for effectively suppressing the formation of vortices in the coolant fluid circulating within the lower plenum of a nuclear reactor where instrumentation conduits are not present in the lower plenum.